1. Field of the Invention
The present invention relates generally to the field of emissions control and, in particular to a new and useful method and/or system by which to control, treat and/or mitigate various liquid-based acidic compounds that are produced during one or more post-combustion phases, or post-combustion processes, of oxy-combustion (e.g., during a compression step and/or cooling step) from various gaseous acid compounds and/or gaseous acid precursor compounds (e.g., SOx, NOx, etc.). In one embodiment, the present invention relates to a method and/or system by which such one or more liquid-based acid compounds are recycled into the flue gases and/or into one or more of the emissions control and/or flue gas treatment equipment of an oxy-combustion power generation system.
2. Description of the Related Art
As is well known, the use of coal for the generation of power has various drawbacks. One such drawback is the creation of carbon dioxide, a greenhouse gas emission, a gas that many believe adds to the growing problem of global warming. In light of this various technologies have been developed to employ carbon capture, utilization or storage (CCUS) as one way to address global climate concerns. In connection with such concerns various technologies for carbon capture have been investigated including oxy-combustion.
As known to those of skill in the art, oxy-fuel combustion (or oxy-combustion) is the process of burning a fuel using an oxidant with less nitrogen than atmospheric air (e.g., a combination of flue gas and oxygen, pure oxygen, or a combination of oxygen and one or more inert gases), instead of air, or atmospheric air, as the primary oxidant. Since the nitrogen component of air is either reduced, or not present, the nitrogen component of the air is either not converted to nitrogen oxides, or in the case of total oxy-combustion not present to be heated.
In the field of power generation research has turned to the use of oxy-combustion for power generation using one or more fossil fuels, or carbonaceous fuels, as the combustion fuel. There is currently research being done in firing fossil-fueled power plants with a nitrogen-depleted gas, or gas mixture, instead of air. In one such proposed process, almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95 percent oxygen and subsequently mixed with, for example, re-circulated flue gas. Firing with pure oxygen can in some circumstances result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. The recycled flue gas (RFG) can also be used to carry fuel into the boiler and to ensure adequate convective heat transfer. Oxy-fuel combustion produces approximately 75 percent less flue gas than air fueled combustion and produces exhaust consisting primarily of CO2 and H2O.
The justification for using oxy-fueled combustion, or oxy-combustion, is to produce a CO2 rich flue gas ready for purification, compression and/or sequestration. Oxy-fuel combustion has significant advantages over traditional air-fired plants. Among the non-limiting advantages are: (i) the mass and volume of the products of combustion, which essentially comprise the flue gas leaving the process, are reduced by approximately 75 percent; (ii) the size of the flue gas compression and purification equipment can be reduced by approximately 75 percent; (iii) the flue gas is primarily CO2, suitable for separation and treatment for use or sequestration via, for example, converting the CO2 into a liquid or supercritical fluid; (iv) the concentration of undesirable constituents in the flue gas is much higher, making separation easier within the process; (v) most of the flue gas impurities (e.g., water and acid gaseous) are condensable which makes compression by separation and cooling possible; (vi) heat of compression can be captured and reused rather than lost in the flue gas; and (vii) because the amount of nitrogen contained in the combustion air is either greatly reduced and/or eliminated, nitrogen oxide production is greatly reduced and/or eliminated.
Economically speaking oxy-combustion costs more than traditional air-fired combustion. This is because oxy-combustion relies on decreasing the amount of nitrogen in the combustion air via various techniques thereby resulting in an increase in the percentage of oxygen present or available in the combustion air. The oxygen separation process requires a significant amount of energy leading to an increase in cost that is justified by the savings realized in the flue gas treatment plant (CPU). For example, cryogenic air separation can consume on the order of 15 percent of the electricity produced by a fossil, or carbonaceous, fuel-fired power station. However, various new technologies such as membranes and chemical looping are being developed that can be used to reduce this cost.
In the realm of coal power, oxy-combustion has the possibility to achieve a near-zero emission coal power plant, including CO2. To capture CO2, there is one pre-combustion method known as Integrated Gasification Combined Cycle (IGCC) and two post-combustion-based technology paths: oxy-combustion (as described above) and CO2 scrubbing. Oxy-combustion is applied to the entire plant process, inherently providing near-zero emissions. CO2 scrubbing can be applied to all or part of the plant emissions.
To understand how such low emission levels are achievable, consider the process schematic in FIG. 1 depicting typical combustion versus oxy-coal combustion. The oxidant for typical combustion is primarily atmospheric air which contains slight more than 78 percent by volume nitrogen and slight less than 21 percent by volume oxygen. This leads to flue gases that typically contain about 68 to about 73 percent nitrogen, about 13 to about 16 percent carbon dioxide, about 5 to about 10 percent water vapor, plus some oxygen and other minor compounds (as measured after flue gas desulfurization has taken place). On the other hand, the oxidant for oxy-combustion is nearly pure oxygen containing around 95 percent or greater oxygen with the remainder being some nitrogen and some argon. To replace the gas volume produced by the nitrogen in typical combustion using air, flue gas is recycled to the boiler. This in turn leads to flue gases being supplied to the compression purification unit (CPU) containing about 70 percent by volume or more carbon dioxide, with the remainder being primarily water, argon, nitrogen and oxygen. Thus, as can be seen from FIG. 1, combustion air is replaced with oxygen from an air separation unit (ASU). Nitrogen that would normally be conveyed with the air through conventional air-fuel firing is essentially excluded. Instead, in this exemplary set-up, a portion of the CO2-rich flue gas is returned back to a conventional pulverizer/burner system, substituting recycled flue gas (primarily CO2) for the nitrogen in the furnace. The CO2 in oxy-combustion impacts furnace operation and heat transfer in ways similar to the nitrogen in an air-fired system. These features allow the technology to be used in retrofit and repowering applications. Oxy-combustion creates a flue gas that is primarily CO2, rather than nitrogen, and includes other products of combustion (although a greatly reduced amount of NOx). The fraction of the flue gas that is not recirculated to the boiler is sent to a compression purification unit (CPU).
The flue gas leaving the boiler is cleaned using conventional particulate and sulfur removal systems is known to those of skill in the art. Remaining particulate is further filtered in the CPU to protect the compressor systems. Primary and polishing scrubbers are used to reduce sulfur and moisture to required levels in the flue gas prior to recycling of a portion of the flue gas to the boiler and sending the remainder to the CPU. The trace amount of SO2 remaining is removed in the CPU. A NOx removal system (such as an SCR or SNCR) is not required as the remaining combustion-generated NOx is almost completely removed as a condensable in the CPU. Mercury is removed in one or more of the scrubbers and/or CPU. To provide pipeline quality CO2 at the exit of the CPU, a small amount of inert constituents must be removed in the CPU. Small quantities of oxygen, nitrogen and argon present in the oxygen from the ASU (typically 95 percent by volume pure oxygen) and from air in-leakage are vented to the atmosphere, along with a very small amount of some remaining combustion products such as carbon monoxide (CO).
In light of the above, various new emissions issues have arisen in connection with oxy-combustion. For example, various constituents present in the flue gas from oxy-combustion will lead to the generation of various liquid-based acidic compounds when the flue gas is subjected to, for example, compression. For example, a wide range of proposed oxy-combustion processes utilize compression (e.g., wet compression) of the flue gas as a step in rendering the carbon dioxide present in the flue gas suitable for storage (or reuse). During the process of compressing the wet flue gas within the CPU one or more liquid-based acidic compounds are, or will be, generated and will have to be treated and/or disposed of. Additionally, other liquid-based acidic compounds that require treatment can be generated during other phases of oxy-combustion such as water scrubbing, cooling, various adsorption and regeneration processes, etc. In the past such waste streams have been treated in a separate waste treatment process and/or system.
Given the above, a need exists in the art for a method and/or system by which to treat and/or control the liquid-based acidic compounds generated during the various post-combustion stages of an oxy-combustion process without the need for a separate waste treatment process and/or system.